The present invention relates to an optical switch circuit for arbitrarily switching the connections between optical transmission paths of multichannel configuration. Specifically, the present invention is concerned with an optical matrix switch circuit in which a plurality of optical switches, each comprising an optoelectronic element and a light modulating element functioning as an electrooptic element, are arranged on the matrix crosspoints.
The substantial realization of recent optical communication systems offers a new concept of the system providing novel functions or services which have not been practiced in the prior art. By way of example, for a device required for such a system, there is an optical switch circuit which switches at a high speed the connections between a large number of optical transmission channels. In the prior art, the optical switch circuit of this kind has typically employed a mechanical switch in which a movable optical element such as prism or lens in the optical transmission channel itself is movably provided. However, by taking into account requirements, e.g., high switching speed, reliability of operation and multichannel scheme etc., it is expected in the future that non-mechanical switch circuits which can be integrated, will be substituted for such a mechanical switch.
As one example of such a non-mechanical switch, there is known an optical switch circuit of an optical splitter-optical gate type as shown in Japanese Patent Application Laid-open No. 97994/83. The configuration and the operation of the optical splitter-optical gate type optical switch circuit mentioned above will be described.
FIG. 4 is a schematic illustration of the operation of the optical splitter-optical gate type optical switch circuit, wherein a two-input, two-output circuit is shown for facilitating the description. Light signals transmitted via input optical transmission channels 11a and 11b are split by optical splitters 12a and 12b, respectively. Split light signals from the optical splitter 12a are inputted to optical gate switches 13a and 13b, respectively. Likewise, split light signals from the optical splitter 12b are inputted to optical gate switches 13c and 13d, respectively. The optical gate switch is defined as a light modulator type switch which turns on or off the passage of a light signal in accordance with a control signal. For example, when the optical gate switch is in an on-state, it allows a light signal to pass therethrough, while when in an off-state, it allows the light signal to be blocked. In this example, a waveguide path at the output is configured as an optical combiner. The light signal which has passed through the optical gate switch 13a or 13c travels via an optical combiner 14a, and then the combined optical signal is outputted as output light 40a. Likewise, the light signal which has passed through the optical gate switch 13b or 13d travels via an optical combiner 14b, and then the combined optical signal is outputted as output light 40b.
Further understanding of the prior art may be obtained from an example of the circuit connection status when a light signal from the input light transmission channel 11a and a light signal from the input light transmission channel 11b are outputted as the light outputs 40b and 40a, respectively. To establish the above-mentioned connection, it is sufficient that the optical gate switches 13a, and 13d are in off-state an and the optical gate switches 13b and 13c are in on-state an. Other arbitrary connections may be realized by combining the on/off states of the optical gate switches 13a, 13b, 13c and 13d.
The optical switch circuit of the structure stated above effects switching operation for a light signal. Accordingly, this circuit is advantageous in that the bandwidth and the quality of a transmission signal are not degraded and problems due to electromagnetic induction and crosstalk etc. do not occur, as compared to a method in which a light signal is converted once into an electric signal thereafter to effect switching. Further, when compared to a switch which switches light paths of a usual waveguide type (e.g. a directional coupler type optical switch based on electro-optical effect), the above-mentioned optical gate switch can be small-sized, thus providing adaptability to multichannel purpose.
However, the optical switch circuit of this type has the drawbacks stated below. The optical splitter-optical gate type switch circuit is configured so as to optically combine output light flows from optical gate switches. For this reason, it is likely that signal light which has failed to be sufficiently extinguished when an optical gate switch is turned off is mixed into the combined output light as a crosstalk component. As the number of transmission channels increases to a great extent, the occurrence of such an undesirable phenomenon also increases accordingly. Therefore, the optical gate switch is required to have a large extinction ratio rather than small size and high speed requirements. Conventional optical gate switches are described in the following articles: (i) a waveguide type switch utilizing electro-optical effect (e.g. Applied Physics Letters vol. 43, p. 998, 1983); (ii) an absorption type light modulator based on Franz-Keldysh effect (e.g. Journal of Applied Physics vol. 47, p. 1523, 1976); (iii) an optical shutter using liquid crystal or ferroelectric crystal (e.g. Proceedings of IEEE vol. 65, p. 143, 1977); and (iv) a device modulating gain and loss by carrier injection into semiconductor (IEEE Journal of Quantum Electronics. vol. QE-19, p. 157, 1983).
The extinction ratio experimentarily confirmed with the above articles (i) to (iii) is approximately 20 to 30 dB, which is unsatisfactory to the requirement for the optical gate switch. On the other hand, with a switch to control current injection into a semiconductor laser, a so-called semiconductor laser diode switch, described in the article (iv), the extinction ratio of 60 to 70 dB can be obtained. However, since this switch utilizes gain mechanism of the semiconductor laser, its characteristics are dependent upon the mode and the wavelength of incident light, and are not adaptable to the switching of light transmitted through multi-mode optical fiber.
As stated above, an optical gate switch acceptable from a practical point of view has not been realized in the prior art, thus failing to make good use of an excellent feature of the optical splitter-optical gate type switch circuit.